, . THE BLAST. FURNACE. '. , ". '
blast. furnace shall not get cold, and in ordinary practice this'can-only be prevented by carrying a slight'excess of fuel "to allows for variations in the air and in the hurden. When the greater! variable—'-the air—is made a constant from hour to -hour/ the: excess may be reduced to a minimum.
The amount of water in the air at different temperatures and at different degrees of humidity is given herewith (see page 48).
SEC",!.....TIj'1—Metallurgical conditions.—In a charcoal blast furnace no sulphur exists in the fuel, and if there is none in the ore the only .problem is to!smelt the iron and to have a. cinder fluid enough to carry away the, ejarthy materials and not fluid enough'to attack the linirg. When .coke is the fuel, a more basic slag > is needed to hold the sulphur, and a higher temperature to keep thjs slag fluid. With too little fuel the slag will not run freely, and the iron will be high in sulphur; while with too much fuel the iron ytill be high in'silicon and the furnace will tend to stick and harig-In short,; the daily work of the furnaceman is to remove sulphur with the least amount of fuel. Many metallurgical conditions are involved in this problem^ among which are the following1: •
•(1:) Th6 amount of;slag. , ':' «.
(2;) The composition of the slag.
(3 )•• The temperature of the furnace. ;
(1) 'Amount of slag: . .
In case the ore is very pure, say with only two per cent, of silica; and the coke and stone, are moderately low in silica, then :it does'-not suffice to add just enough lime to satisfy the silica, because riot-enough cinder will be produced to carry away the sulphur of the coke, so that silicious materials or impure ores must be added. The same course of procedure may be necessary even in moderately silicious ores, when they contain abnormal amounts of sulphur.
(2) Composition of the slag:
The basis of every cinder is silicate of lime, the silica coming, from the,ore and ash, and the lime from the stone, but there are always other..elements present. Alumina and magnesia are invariably found in.the ore or in the stone, or in both, and they constitute a considerable proportion of the slag, and vary within wide limits. The allowable proportion of magnesia is in doubt.